1. Technical Field
The present invention relates to a system for carrying out a reference point return in a numerical control device, and more particularly, to a reference point return system by which an adjustment of a deceleration dog on a machine side is simplified.
2. Background Art
Numerical control devices have a reference point return function of returning a table or the like of a machine tool to a reference point, whereby the table or the like is manually moved toward the reference point after the power is turned on, and is decelerated in accordance with a signal from a deceleration limit switch. After the deceleration is ended, the table or the like is then moved at a constant low speed (decelerated speed), and is stopped at a point in an electrical grid at which the deceleration limit switch loses contact with the deceleration dog and thus is closed.
FIG. 4 is a diagram of a conventional reference point return system wherein, when a machine table 1 to which a deceleration dog 3a is secured is moved in the direction indicated by the arrow in the figure, a deceleration limit switch 2 is operated, and when a deceleration signal DEC is turned "off", a reference point return processing means 5a of a numerical control device (CNC) 4a reduces the speed of the machine table 1 by decelerating a servomotor, not shown. Thereafter, the machine table 1 is moved at a constant low speed, and when it reaches a point in an electrical grid at which the deceleration dog 3a has moved past the deceleration limit switch 2, and accordingly the deceleration signal is turned "on", the processing means stops the machine table at that point to thereby end the reference point return process.
The contact of the deceleration limit switch is a break contact, and therefore, the on/off switching thereof is converse to the contact/non-contact with the deceleration dog. Namely, while the deceleration dog 3a is running over the deceleration limit switch 2, the deceleration signal DEC is off.
The conventional reference point return system requires a deceleration dog having a considerable length, and this length must be set in accordance with the deceleration distance. Furthermore, an adjustment is required so that the deceleration limit switch 2 is turned on when the trailing end of the deceleration dog is located between electrical grid points.
This adjustment is extremely minute, because the pitch of the electrical grid is about several millimeters to 10 mm, and a small misadjustment may cause a deviation of one pitch. Moreover, if the deceleration dog is located under the machine table 1, for example, an external cover and the like must be removed to enable an accurate adjustment of the position of the deceleration dog, thus making assembling and maintenance complicated.